1. Field of the Invention
The invention relates to a voltage supply circuit provided with a feedback type voltage supply or source, and in particular, to a voltage supply circuit suitable for use in supplying a given voltage to a load through which an operating or working current having a peak value greater than a steady-state current flows upon inversion of its operation.
2. Description of the Related Art
A general arrangement of an example of the feedback type voltage supply or source which has been used in the prior art is shown in FIG. 3. The illustrated feedback type voltage supply 10 comprises a first operational amplifier 11 having a non-inverting input terminal which is connected to a point of a common potential (ground) and an inverting input terminal which is supplied with a constant voltage Vin from a voltage source 18, a voltage output terminal TO to which a voltage V.sub.0 outputted from the operational amplifier 11 is fed through a current measuring resistor 13, a negative feedback circuit connected between the voltage output terminal TO and the inverting input terminal of the operational amplifier 11, the negative feedback circuit including a second operational amplifier 12, a switching element 14 connected in parallel with the resistor, and a phase compensation capacitor 15 also connected in parallel with the resistor 13.
The voltage V.sub.0 given to the output terminal TO is supplied to a load 25 as a given operating or working voltage, and is also negatively fed back to the inverting input terminal of the first operational amplifier 11 through the second operational amplifier 12. The voltage source 18 which feeds the constant voltage Vin to the inverting input terminal of the operational amplifier 11 is generally formed by a D/A (digital-analog) converter so that the magnitude of the constant voltage Vin fed to the inverting input terminal of the operational amplifier 11 can be set to an arbitrary value depending upon a digital value given to the D/A converter.
The feedback type voltage supply 10 constructed in the manner mentioned above is often used, as a power supply or source, in a semiconductor device testing apparatus (commonly called IC tester) for testing various kinds of semiconductor devices such as IC memories, for instance, each formed into a semiconductor integrated circuit (hereinafter referred to as IC), the power supply supplying a predetermined operating voltage to semiconductor devices to be tested on testing them.
As illustrated in FIG. 3, the feedback type voltage supply 10 is used in a current measuring circuit for measuring a current flow through a semiconductor device under test as a power supply for applying a given operating voltage to the semiconductor device under test. For this end, there is provided current measuring means 20 including a differential amplifier 21 for taking out a voltage developed across the current measuring resister 13 connected between the first operational amplifier 11 and the voltage output terminal TO, and an A/D (analog-digital) converter 22 for converting a voltage value detected by the differential amplifier 21 into a digital value. However, it should be noted that this shows merely an exemplary one and that the circuit arrangement of the feedback type voltage supply 10 may be changed or modified depending on its application.
When the load 25 which is connected between the output terminal TO of the Voltage supply 10 and the common potential point is a semiconductor integrated circuit having a complementary MOS structure (hereafter referred to as CMOS type IC), for example, a current IL flowing through CMIOS type IC changes in a manner illustrated in FIG. 4A each time an active element (field effect transistor) or elements within the CMOS type IC inverts or invert in its or their operation. That is, during a steady-state operation of the IC in which no inverting operation of active elements occurs, a steadystate current .DELTA.I of very small magnitude flows therethrough, and during inverting operation thereof, an operating current IP having a very large magnitude flows therethrough, and upon completion of the inverting operation, the current is restored to the steady-state current .DELTA.I of very small magnitude, again. The ratio of the operating current IP to the steady-state current .DELTA.I is very high, on the order of 1000:1, for example.
Even if the load 25 is not a CMOS type IC, the current flowing through the load 25 changes in the similar manner as illustrated in FIG. 4A where it is one through which an operating current having a peak value greater than the magnitude thereof in the steady-state flows when there occurs an inversion in operation thereof. As a consequence, the voltage V.sub.0 given to the voltage output terminal TO from the voltage supply 10 changes in a manner depicted in FIG. 4B, requiring a significant length of time until it is restored to the original steady-state voltage Vin.
As is recognized, an increase in the operating speed of IC is being ever demanded recently, and with an IC which operates at a higher speed or rate, its inverting operation takes place more quickly. It will then be seen that the period during which the operating current IP with a higher magnitude flows is significantly shortened. As the period during which the operating current IP flows is reduced in time, the time interval during which the steady-state current .DELTA.I of very small magnitude flows becomes shortened, resulting in a situation that the operating current IP of a high magnitude occurs again before the steady-state current .DELTA.I can be stabilized. Accordingly, for an IC having a high operation speed, it follows that the current measurement must be made before the steady-state current is stabilized, making it difficult to measure the steady-state current exactly. In addition to the measurement of the steady-state current, there is also a need to return the voltage V.sub.0 supplied to the voltage output terminal TO of the voltage supply 10 to the original steady-state voltage Vin quickly to assure a stable operation (or to increase the reliability) of the IC which is operating at a high speed.
For this end, in the current measuring circuit shown in FIG. 3, a bypass capacitor 16 is connected between the output terminal TO and the common potential point (and thus in parallel with the load 25), and is normally charged to the steady-state voltage V.sub.0 (and accordingly, to the steady-state voltage Vin), the arrangement being such that when the operating current IP flows through the load 25, the switching element 14 is turned on to bypass the resister 13, thus allowing an increased current flow. Subsequent to the flow of the operating current IP, the switching element 14 is turned off to allow a measurement of the steady-state current .DELTA.I.
However, in order to turn the switching element 14 on as the operating current IP flows and to turn it off subsequent to the flow of the operating current IP, it is necessary that the turn on/off of the switching element 14 be effected in accordance with a change in the current IL flowing through the load 25 in real time, thus presenting a difficulty that a complicated control is required.
As the current IL flows through the load 25, a change in the voltage V.sub.0 at the output terminal TO of the voltage supply 10 (namely, a load fluctuation characteristic) is determined by the magnitude of the current IL, the capacitance of the bypass capacitor 16 and the response of the voltage supply 10. To obtain a rapid load fluctuation characteristic, it is necessary to increase the frequency response of the voltage supply 10, thus accelerating the response. On the other hand, it is necessary to choose a small capacitance for the bypass capacitor 16 in order to increase the frequency response of the voltage supply 10.
As the operating current IP flows, the initial current is fed to the load 25 from the bypass capacitor 16. If the bypass capacitor 16 has a reduced capacitance, the reduced stored charge causes an increased initial voltage change .DELTA.V.sub.0 in the voltage V.sub.0 at the output terminal TO. In such case, because of the increased frequency response of the voltage supply 10, a time interval during which the voltage V.sub.0 continues to change (or a time interval from the beginning of a change in the voltage V.sub.0 until it resumes the original steady-state voltage Vin) is relatively short. However, there is a limit on the degree to which the response of the voltage supply 10 can be accelerated with a reduction in the capacitance of the bypass capacitor 16 alone. It then follows that the circuit cannot be used as a power supply for a load which operates with a degree of rapidness.
By contrast, when an increased capacitance is chosen for the bypass capacitor 16, the initial voltage change .DELTA.V.sub.0 in the voltage V.sub.0 at the output terminal TO can be reduced as the operating current IP flows. However, because the frequency response of the voltage supply 10 is decreased, its response becomes retarded, resulting in an increased length of time during which the voltage V.sub.0 continues to change. Hence, it cannot be used as a power supply for a load which operates rapidly.